Metals are commonly extracted from their ores by processes in which the minerals are concentrated and the concentrate roasted to convert complex or otherwise difficult-to-extract metal compounds into compounds which can more easily be separated for recovery of the metal content.
In one such process, applicable to sulfide minerals, sulfides are converted by roasting into water soluble sulfates which may readily be recovered by leaching with water.
It is generally agreed that the mechanism involved in converting sulfides to sulfates proceeds via an oxide as follows: EQU MS+3/2 O.sub.2 .fwdarw.MO+SO.sub.2 ( 1) EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 ( 2) EQU MO+SO.sub.3 .fwdarw.MSO.sub.4 ( 3)
wherein M represents metal. (See Palperi, M. and Aaltonen, O., Sulfatizing Roasting and Leaching of Cobalt Ores at Outokumpu Oy, Journal of Metals, February 1971, pp. 34-38.)
For the sulfatization process to be economical, sufficiently high percentages of the metals must be converted under reasonable operating conditions to easily extractable water soluble sulfates while other, undesired materials remain insoluble. In particular, it is important that the iron content of the ore remains insoluble so that it can easily be disposed of as a solid by-product, rather than being leached as a water soluble product which would require subsequent, difficult separation and disposal steps.
Some of the desired non-ferrous metals in sulfide ores can be difficult to recover as water soluble sulfates in the sulfatization process. Nickel in particular is difficult to sulfatize efficiently. Attempts to sulfatize nickel sulfides were unsuccessful until Thornhill, in U.S. Pat. Nos. 2,813,015 and 2,813,016, showed that the addition of sodium sulfate to the roaster feed promoted the sulfatization of nickel sulfide. The sodium sulfate is used to control particle size in a fluidized bed and is said to render unstable the nickel ferrites in pyrrhotite according to the following equation: EQU NiFe.sub.2 O.sub.4 +Na.sub.2 SO.sub.4 .fwdarw.Na.sub.2 Fe.sub.2 O.sub.4 +NiSO.sub.4 ( 4)
These patents also describe sodium sulfate taking part in reactions providing sodium pyrosulfate, a strong sulfating agent, and sulfur trioxide for sulfatization of the metal oxide: EQU Na.sub.2 SO.sub.4 +SO.sub.3 .fwdarw.Na.sub.2 S.sub.2 O.sub.7 ( 5) EQU NiO+SO.sub.3 .fwdarw.NiSO.sub.4 ( 6)
Thus, these patents demonstrate the addition of a sulfate material as a requirement to enhance the sulfatization of nickel. While the use of sodium sulfate in the roast does provide significant improvement in the amount of metal extracted as water soluble sulfate, the percentage of such metal extracted from the raw material needs to be increased for improved process economics. Unfortunately, the addition of sodium sulfate to the roaster increases the sulfur content of the roast and contributes to the generation of sulfur dioxide and sulfur trioxide gases during the roasting procedure. This is undesirable because these gases are subject to pollution control regulations so that increasing their quantity in the off gases increases the cost of pollution control and thereby also increases the cost of the extraction process.
Attempts have been made to decrease the amount of toxic sulfur gases liberated in the sulfatization process. In U.S. Pat. No. 3,791,812, copper, nickel, cobalt and manganese are extracted as water soluble salts from sulfide ores by roasting the ore in the presence of sodium chloride. The use of sodium sulfate is avoided and the amount of sulfur dioxide in the off gases is reduced by conducting the roasting in at least two stages and using sulfur dioxide liberated in the first stage as a sulfatizing agent in a subsequent stage. Unfortunately, iron as well as the non-ferrous metals is extracted as a water soluble salt, necessitating the expense of further separation steps to remove the iron.
Other alkali metal salts besides sodium sulfate and chloride have been used in the extraction of non ferrous metals from their ores. U.S. Pat. No. 2,775,517 discloses a process for separating nickel from low sulfur content iron oxide ores. The ore is roasted with alkali, such as sodium hydroxide, carbonate or bicarbonate and the roasted product leached with water to extract chromium and aluminum. The nickel remains in a water insoluble state and is subsequently removed by treatment in an autoclave with ferric or ferrous chloride or sulfate. This process is not a sulfatization process because the raw material is not a sulfide ore. The process suffers from the disadvantages that the nickel is not converted into a readily-leachable, water soluble salt and that an autoclave is required, thereby increasing plant and operating costs and processing complexity.
There is, therefore, a need for a process for extracting non-ferrous metals from sulfide ores by sulfatization of such metals into readily leachable salts which extracts high percentages of the metals in the ore in an economical and environmentally acceptable process.